U.S. patent application number 10/828507 was filed with the patent office on 2005-01-06 for preparation and storage of membrane and electrode assemblies.
Invention is credited to Murphy, Oliver J., Salinas, Carlos.
Application Number | 20050000799 10/828507 |
Document ID | / |
Family ID | 33310817 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050000799 |
Kind Code |
A1 |
Murphy, Oliver J. ; et
al. |
January 6, 2005 |
Preparation and storage of membrane and electrode assemblies
Abstract
Protecting a membrane and electrode assembly in an
electrochemical cell having one or more electrocatalysts in
intimate contact with the membrane during storage or shipment of
the cell. The membrane may be provided in either the non-proton
form of a dry or hydrated cation exchange membrane, such as an
alkali metal cation form or an ammonium cation form; the wet or dry
precursor form of a cation exchange membrane, such as the
non-ionically conducting sulfonyl-fluoride polymer membrane; or the
dry proton form of a cation exchange membrane. These membrane
surfaces are not acidic under open circuit conditions experienced
during storage or shipment of the cell. Since some electrocatalysts
are degraded during contact with the acidic surface of a hydrated
membrane, the non-acidic surface of the membrane protects these
electrocatalysts. The method may be used on newly assembled
electrochemical cells, on cells being taken out of service, and on
membrane and electrode assemblies.
Inventors: |
Murphy, Oliver J.; (Bryan,
TX) ; Salinas, Carlos; (Bryan, TX) |
Correspondence
Address: |
STREETS & STEELE
13831 NORTHWEST FREEWAY
SUITE 355
HOUSTON
TX
77040
US
|
Family ID: |
33310817 |
Appl. No.: |
10/828507 |
Filed: |
April 19, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60463768 |
Apr 17, 2003 |
|
|
|
Current U.S.
Class: |
204/252 ;
204/253; 204/291; 204/296; 502/101 |
Current CPC
Class: |
H01M 8/0289 20130101;
H01M 8/0271 20130101; Y10T 29/49108 20150115; H01M 8/1004 20130101;
H01M 4/881 20130101; C25B 9/23 20210101; Y02E 60/50 20130101; H01M
4/90 20130101; H01M 8/04067 20130101 |
Class at
Publication: |
204/252 ;
502/101; 204/296; 204/253; 204/291 |
International
Class: |
C25B 009/08; C25B
011/06; C25B 013/08 |
Claims
What is claimed is:
1. A method, comprising: providing a membrane and electrode
assembly having one or more electrocatalysts in intimate contact
with a cation exchange membrane in an alkali metal cation form
during a period without an electrical current passing through the
membrane and electrode assembly.
2. The method of claim 1, wherein at least one of the one or more
electrocatalysts has an electrocatalytic activity that diminishes
when placed in contact with a hydrated proton form of the cation
exchange membrane in the absence of an electrical current flowing
through the cell.
3. The method of claim 2, wherein the electrocatalytic activity
diminishes by a process selected from degradation, dissolution,
corrosion, or a combination thereof.
4. The method of claim 1, wherein the one or more electrocatalysts
includes lead dioxide.
5. The method of claim 1, further comprising: placing the membrane
and electrode assembly into an electrochemical cell or an
electrochemical cell stack.
6. The method of claim 5, wherein the electrochemical cell or
electrochemical cell stack includes a fuel cell or an electrolytic
cell.
7. The method of claim 1, wherein the cation exchange membrane is
hydrated.
8. The method of claim 1, wherein the cation exchange membrane is
dry.
9. The method of claim 1, further comprising: supplying the
membrane and electrode assembly with reactants; and providing an
electrical current through the membrane and electrode assembly to
liberate protons and convert the cation exchange membrane from the
alkali metal cation form to an acidic proton form.
10. The method of claim 1, further comprising: placing the membrane
and electrode assembly into an electrochemical cell or an
electrochemical cell stack.
11. The method of claim 10, wherein the cation exchange membrane is
wet during the placing of the membrane and electrode assembly into
the electrochemical cell or the electrochemical cell stack.
12. The method of claim 10, wherein the cation exchange membrane is
dry during the placing of the membrane and electrode assembly into
the electrochemical cell or the electrochemical cell stack.
13. A method, comprising: providing a membrane and electrode
assembly having one or more electrocatalysts in intimate contact
with a cation exchange membrane in a dry proton form during a
period without an electrical current passing through the membrane
and electrode assembly.
14. The method of claim 13, wherein at least one of the one or more
electrocatalysts has an electrocatalytic activity that diminishes
when placed in contact with a hydrated proton form of the cation
exchange membrane in the absence of an electrical current flowing
through the cell.
15. The method of claim 14, wherein the electrocatalytic activity
diminishes by a process selected from degradation, dissolution,
corrosion, or a combination thereof.
16. The method of claim 13, wherein the one or more
electrocatalysts includes lead dioxide.
17. The method of claim 13, further comprising: placing the
membrane and electrode assembly into an electrochemical cell or an
electrochemical cell stack.
18. The method of claim 17, wherein the electrochemical cell or
electrochemical cell stack includes a fuel cell or an electrolytic
cell.
19. The method of claim 13, further comprising: supplying the
membrane and electrode assembly with water to convert the proton
exchange membrane from the dry proton form to a hydrated acidic
form.
20. The method of claim 13, further comprising: placing the
membrane and electrode assembly into an electrochemical cell or
electrochemical cell stack in a dry condition.
21. The method of claim 13, further comprising: supplying the
membrane and electrode assembly with water to convert the proton
exchange membrane from the dry proton form to a hydrated acidic
form; and providing an electrical current through the membrane and
electrode assembly to liberate protons and convert the proton
exchange membrane from the proton non-conducting form to a wet acid
form.
22. The method of claim 21, wherein the steps of supplying water
and providing an electrical current are performed simultaneously or
nearly simultaneously.
23. The method of claim 20, wherein the electrochemical cell or
electrochemical cell stack is selected from an electrolyzer and a
fuel cell.
24. A method, comprising: providing a membrane and electrode
assembly having one or more electrocatalysts in intimate contact
with a cation exchange membrane in a sulfonyl-fluoride form during
a period without an electrical current passing through the membrane
and electrode assembly.
25. The method of claim 24, wherein at least one of the one or more
electrocatalysts has an electrocatalytic activity that diminishes
when placed in contact with a hydrated proton form of the cation
exchange membrane in the absence of an electrical current flowing
through the cell.
26. The method of claim 25, wherein the electrocatalytic activity
diminishes by a process selected from degradation, dissolution,
corrosion, or a combination thereof.
27. The method of claim 24, wherein the one or more
electrocatalysts includes lead dioxide.
28. The method of claim 24, further comprising: placing the
membrane and electrode assembly into an electrochemical cell or an
electrochemical cell stack.
29. The method of claim 28, wherein the electrochemical cell or
electrochemical cell stack includes a fuel cell or an electrolytic
cell.
30. The method of claim 24, further comprising: converting the
cation exchange membrane from the sulfonyl-fluoride form to an
alkali metal cation form.
31. The method of claim 30, wherein the step of converting includes
contacting the cation exchange membrane with an alkali metal
hydroxide solution.
32. The method of claim 31, wherein the alkali metal hydroxide is
selected from NaOH, KOH, LiOH, RbOH, CsOH, FrOH, and combinations
thereof.
33. The method of claim 31, wherein the alkali metal hydroxide
solution has a concentration of between about 0.1M and about
10M.
34. The method of claim 31, wherein the alkali metal hydroxide
solution has a concentration of between about 0.5M and about
5M.
35. The method of claim 31, wherein the alkali metal hydroxide
solution has a concentration of between about 0.75M and about
3M.
36. The method of claim 31, wherein the alkali metal hydroxide
solution contacts the proton exchange membrane for between about
0.25 hours and about 24 hours.
37. The method of claim 31, wherein the alkali metal hydroxide
solution contacts the proton exchange membrane for between about
0.5 hours and about 12 hours.
38. The method of claim 31, wherein the alkali metal hydroxide
solution contacts the proton exchange membrane for between about 1
hour and about 6 hours.
39. The method of claim 30, further comprising: providing the
cation exchange membrane in a wet state.
40. The method of claim 30, further comprising: providing the
cation exchange membrane in a dry state.
41. The method of claim 30, further comprising: assembling the
membrane and electrode assembly into an electrochemical cell or an
electrochemical cell stack.
42. The method of claim 41, wherein the cation exchange membrane is
wet during the assembling of the membrane and electrode assembly
into the electrochemical cell or the electrochemical cell
stack.
43. The method of claim 41, wherein the cation exchange membrane is
dry during the assembling of the membrane and electrode assembly
into the electrochemical cell or the electrochemical cell
stack.
44. The method of claim 30, further comprising: supplying the
membrane and electrode assembly with reactants; and providing an
electrical current through the membrane and electrode assembly to
liberate protons and convert the cation exchange membrane from the
alkali metal cation form to an acidic proton form.
45. The method of claim 44, wherein the steps of supplying
reactants and providing an electrical current are performed
simultaneously or nearly simultaneously.
46. A method, comprising: passing electrical current through a
membrane and electrode assembly having one or more electrocatalysts
in intimate contact with a cation exchange membrane in a hydrated
proton form; and then supplying an alkali metal hydroxide solution
to a membrane and electrode assembly under an electrical potential;
and then removing an electrical potential from across the membrane
and electrode assembly, wherein the proton exchange membrane is
converted from the hydrated proton form to the alkali metal cation
form.
47. The method of claim 46, further comprising: removing the alkali
metal hydroxide solution from the electrochemical cell.
48. The method of claim 46, further comprising: supplying the
electrochemical cell with reactants; and restoring the electrical
potential across the electrochemical cell to liberate protons and
convert the proton exchange membrane from the alkali metal cation
form back to the acidic form.
49. The method of claim 46, wherein the alkali metal hydroxide is
selected from NaOH, KOH, LiOH, RbOH, CsOH, FrOH and combinations
thereof.
50. The method of claim 49, wherein the alkali metal hydroxide
solution has a concentration of between about 0.1M and about
10M.
51. The method of claim 49, wherein the alkali metal hydroxide
solution has a concentration of between about 0.5M and about
5M.
52. The method of claim 49, wherein the alkali metal hydroxide
solution has a concentration of between about 0.75M and about
3M
53. The method of claim 45, wherein the step of converting the
proton exchange membrane further comprises: contacting the proton
exchange membrane with the alkali metal hydroxide solution for a
period between about 0.25 hour and about 24 hours.
54. The method of claim 53, wherein the period is between about 0.5
hours and about 12 hours.
55. The method of claim 53, wherein the period is between about 1
hour and about 6 hours.
56. A device, comprising: a membrane and electrode assembly having
one or more electrocatalysts in contact with a membrane in an
alkali metal cation form, wherein at least one of the one or more
electrocatalysts is characterized by an electrocatalytic activity
that diminishes when placed in contact with a hydrated proton form
of the cation exchange membrane in the absence of an electrical
current through the cell.
57. The device of claim 56, wherein the membrane and electrode
assembly is assembled into an electrochemical cell.
58. The device of claim 57, wherein the electrochemical cell is
selected from a fuel cell and an electrolytic cell.
59. The device of claim 57, wherein the membrane and electrode
assembly is disposed between an anode flow field and a cathode flow
field.
60. The device of claim 57, wherein the electrochemical cell is
disposed in an electrochemical cell stack.
61. The device of claim 56, wherein the one or more
electrocatalysts includes lead dioxide.
62. A device, comprising: a membrane and electrode assembly having
one or more electrocatalysts in contact with a membrane in a
sufonyl-fluoride form, wherein at least one of the one or more
electrocatalysts is characterized by an electrocatalytic activity
that diminishes when placed in contact with a hydrated proton form
of the cation exchange membrane in the absence of an electrical
current through the cell.
63. The device of claim 62, wherein the membrane and electrode
assembly is assembled into an electrochemical cell.
64. The device of claim 63, wherein the electrochemical cell is
selected from a fuel cell and an electrolytic cell.
65. The device of claim 64, wherein the membrane and electrode
assembly is disposed between an anode flow field and a cathode flow
field.
66. The device of claim 63, wherein the electrochemical cell is
disposed in an electrochemical cell stack.
67. The device of claim 62, wherein the one or more
electrocatalysts includes lead dioxide.
68. A device, comprising: a membrane and electrode assembly having
one or more electrocatalysts in contact with a membrane in a dry
proton form, wherein at least one of the one or more
electrocatalysts is characterized by an electrocatalytic activity
that diminishes when placed in contact with a hydrated proton form
of the cation exchange membrane in the absence of an electrical
current through the cell.
69. The device of claim 68, wherein the membrane and electrode
assembly is assembled into an electrochemical cell.
70. The device of claim 69, wherein the electrochemical cell is
selected from a fuel cell and an electrolytic cell.
71. The device of claim 69, wherein the membrane and electrode
assembly is disposed between an anode flow field and a cathode flow
field.
72. The device of claim 69, wherein the electrochemical cell is
disposed in an electrochemical cell stack.
73. The device of claim 68, wherein the one or more
electrocatalysts includes lead dioxide.
Description
[0001] This application claims priority of U.S. provisional patent
application 60/463,768 filed on Apr. 17, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to ionically conducting
membranes and in particular to membrane and electrode assemblies
for use in electrochemical cells, and in electrochemical cell
stacks.
[0004] 2. Description of the Related Art
[0005] Central to the operation of any electrochemical cell, such
as a fuel cell or an electrolytic cell, is the occurrence of
oxidation and reduction reactions that produce or consume
electrons. These reactions take place at catalyzed
electrode/solution interfaces, where the electrodes must be good
electronic conductors. In operation, a cell is connected to an
external load (in the case of a fuel cell) or to an external
voltage source (in the case of an electrolytic cell), and electrons
transfer electric charge between the anode and the cathode through
the external circuit. To complete the electrical circuit through
the cell, an additional mechanism must exist for internal charge
transfer. One or more electrolytes, usually in the form of a
solution or ion exchange membranes, provide internal charge
transfer by ionic conduction. These same electrolytes must be poor
electronic conductors to prevent internal short-circuiting of the
cell.
[0006] Polymeric ion exchange membranes comprise a broad category
of thin solid polymer electrolyte materials that can be used when
solvated in electrochemical cells to facilitate ionic conduction
between an anode and cathode within an electrochemical cell. Cation
exchange membranes allow the transport of cations through
membranes, while anion exchange membranes allow the transport of
anions through membranes. Cation exchange membranes are most
suitable for use in electrochemical cells in either the proton (or
acid) form, or in the alkali metal cation (or salt) form. When a
solid polymer electrolyte membrane is in the proton, or acid, form
the surface and bulk of the membrane, when in contact with pure
water has a pH of about one. However, when a solid polymer
electrolyte membrane is in the alkali metal cation, or salt, form,
the surface and bulk of the membrane when in contact with pure
water has a pH of about seven, that is a neutral pH value.
[0007] Proton exchange membranes (PEM's) are one category of cation
exchange membranes or solid polymer electrolytes that are
particularly suitable for use in conjunction with electrochemical
cells. PEM's typically have a polymer matrix with functional groups
attached that are capable of exchanging cations or protons. The
polymer matrix in one instance can consist of an organic polymer
such as polystyrene, or other polytetrafluoroethylene (PTFE)
analog. In general, the PEM material is an acid with sulfonic acid
groups, carboxylic acid groups, or other acidic functional groups
incorporated into the organic polymer chains that make up the
polymer matrix.
[0008] The apparent advantages of using ion exchange membranes in
electrochemical cells are numerous. The solid electrolyte membranes
are simpler to use and more compact than other types of
electrolytes, e.g., aqueous solutions, molten salts, etc. In
addition, the use of an ion exchange membrane instead of a liquid
electrolyte offers several advantages, such as simplified fluid
management and elimination of the potential of corrosive liquids.
In systems using an ion exchange membrane, the membrane also serves
as an electronically insulating separator between the anode and
cathode of an electrochemical cell.
[0009] Conventional cation conducting membranes for use in
electrochemical cells consist of homogeneous polymer materials. The
chemical formula for exemplary cation exchange polymers used to
form polymer electrolyte membranes for use in electrochemical cells
are presented in FIG. 1. These polymers represent a class of
compounds known as perfluorosulfonic acids (PFSA), which are fully
fluorinated, i.e., fluorine atoms have replaced all of the sites
usually occupied by hydrogen atoms in a hydrocarbon polymer. This
makes the polymers extremely resistant to chemical attack. These
perfluorosulfonic acid (PFSA) polymers are used in proton exchange
membranes such as DuPont's NAFION.RTM. 115, where n.about.6.5 and
m=1, and the Dow Chemical membrane, where n.about.6 and m=0.
[0010] As shown in FIG. 2, PFSA polymers are generally synthesized
by the copolymerization of a derivatized, or active, co-monomer
with tetrafluoroethylene, (TFE). The sulfonate functionalities
(R--SO.sub.3.sup.-) provide a stationary counter charge for mobile
cations (H.sup.+, Li.sup.+, Na.sup.+, etc.), which are generally
monovalent. The mobile cations are conducted through the membrane
material from one sulfonate group to the next sulfonate group, as
illustrated in FIG. 3, which is a schematic drawing of the commonly
proposed structure for perfluorosulfonic acid (PFSA) polymers, as
typified by NAFION.RTM. (a registered trademark of Dupont of
Wilmington, Del.).
[0011] A sheet of conventional polymer material in the non-ionic
sulfonyl fluoride form (formulated as R--SO.sub.2F) is first
hydrolyzed with a strong base solution, such as NaOH, to convert
the polymer to the ionomeric form (see FIG. 2), and then treated
with a strong acid solution to convert the polymer to the acid
form. The significant reactions are illustrated by equations (1)
and (2).
R--SO.sub.2F+2NaOH.fwdarw.R--SO.sub.3Na+NaF+H.sub.2O (1)
R--SO.sub.3Na+H.sub.2SO.sub.4.fwdarw.R--SO.sub.3H+NaHSO.sub.4
(2)
[0012] In an electrochemical cell, the cation exchange membrane
polymer electrolyte is typically used in the acid, or proton, form,
exhibiting a strongly acidic surface when water, or a mixed
aqueous/organic solvent (e.g., water-methanol), or an organic
solvent (e.g., methanol) is introduced into the electrochemical
cell and makes contact with the proton exchange membrane (PEM). On
applying a DC electrical potential across the cell, which causes a
DC current to flow through the cell, the protons are driven from
the PEM/anode interface through the PEM toward the PEM/cathode
interface. The anode electrocatalysts and cathode electrocatalysts
are in intimate contact with the proton exchange membrane and, in
many cases, the electrocatalysts are deposited directly upon the
proton exchange membrane. This intimate contact exposes the anodic
and cathodic electrocatalysts to the acidic surfaces of the
hydrated proton exchange membrane in the acidic proton form. For
some electrocatalyst materials, such as some elemental metals,
metal alloys, metal oxides, metal borides, metal nitrides, and
metal carbides, whether supported or unsupported on a catalyst
substrate, this intimate contact can give rise to situations where
the electrocatalyst materials, and/or their supports are chemically
unstable, and/or they become inactive from an electrocatalytic
perspective. Chemical instability, or loss of electrocatalytic
activity, of electrocatalyst materials physically in contact with
solvated proton exchange membranes arises from chemical reactions
between the electrocatalyst materials and the solvent, which is
usually water, in a strong acid environment in the absence of
cathodic or anodic protection associated with a suitably applied DC
cell voltage. These chemical reactions can include electrocatalyst
dissolution processes, electrocatalyst dissolution/reprecipitation
processes, electrocatalyst corrosion processes, film formation
processes, electrocatalyst decomposition processes, etc. The rates
of reaction of many of these processes are accelerated considerably
in the strongly acidic environment of a hydrated proton exchange
membrane.
[0013] For any given electrocatalyst material, whether supported or
unsupported on an electrocatalyst support material, which is in
intimate contact with a solvated (usually hydrated) proton exchange
membrane, the nature and magnitude of the electrode potential
applied across the electrocatalyst/hydrated proton exchange
membrane interface controls the rate of the electrocatalyst
degradation processes outlined above. An electrode potential across
an electrocatalyst/hydrated proton exchange membrane interface is
created when a voltage from a DC power supply is applied to an
electrochemical cell comprising a cathode electrocatalyst/hydrated
proton exchange membrane/anode electrocatalyst combination, where
each electrocatalyst component is in intimate contact with opposing
sides of the hydrated proton exchange membrane. This causes a DC
current to flow in the circuit that includes the electrochemical
cell and the DC power supply. To support the flow of DC current an
anodic electrochemical oxidation reaction (e.g., oxygen and/or
ozone evolution) takes place at the anode electrocatalyst/hydrated
proton exchange membrane interface and a cathodic electrochemical
reduction reaction (e.g., hydrogen evolution) takes place at the
cathode electrocatalyst/hydrated proton exchange membrane
interface. Thus, the nature of the electrode potential (anodic or
cathodic) and the magnitude of the electrode potential whether in
the hydrogen evolution region, that is, whether it is less than
0.000V (SHE; standard hydrogen electrode), in the region of
electrochemical stability of water, that is, between 0.000 V (SHE)
and 1.23 V (SHE), or in the oxygen/ozone evolution region, that is,
greater than 1.23 V (SHE) have a dominant influence on the
degradation processes taking place at electrocatalyst/hydrated
proton exchange membrane interfaces.
[0014] For purposes of illustration, the influence of the nature
and magnitude of the electrode potential applied across an
electrocatalyst/hydrated proton exchange membrane interface will be
demonstrated for elemental copper, which is known to be a hydrogen
evolution electrocatalyst, using the electrode potential--pH
equilibrium diagram for the system copper-water at 25.degree. C. in
aqueous solutions (FIG. 4) and for a metal oxide, lead dioxide
(PbO.sub.2), which is known to be an ozone evolution
electrocatalyst, using the electrode potential--pH equilibrium
diagram for the system lead-water at 25.degree. C. (FIG. 5). In
both Figures, for any solution pH, the electrode potential domain
between the dashed lines indicated by `a` and `b` corresponds to
those potentials where water is electrochemically stable. Below the
dashed line indicated by `a`, hydrogen evolution from water can
take place. However, the magnitude of the cathodic, or negative,
electrode potential necessary to be applied to induce hydrogen
evolution depends on the electrocatalytic activity of the
particular electrocatalyst used. Similarly, above the dashed line
indicated by `b`, oxygen evolution from water can take place.
However, the magnitude of the anodic, or positive, electrode
potential necessary to be applied to induce oxygen evolution
depends on the electrocatalytic activity of the particular
electrocatalyst used.
[0015] Typically, hydrated proton exchange membranes, that is, in
the acidic proton form, such as NAFION.RTM. supplied by Du Pont,
Wilmington, Del., have an effective pH of about 1. It can be seen
from FIG. 4 that copper used as an electrocatalyst in any acidic
aqueous environment having a pH of about 1 is only stable under
cathodic electrode potentials corresponding to hydrogen evolution.
At potentials more positive than approximately 0.000 V (SHE),
copper metal dissolves to give Cu.sup.+ and Cu.sup.++ cations.
[0016] It is well known that an anodic, or positive, electrode
potential of at least 1.500 V (SHE) must be applied to an
electrocatalyst/aqueous solution interface, where the aqueous
solution or environment has a pH of about 1, to evolve ozone gas
from water. It can be seen from FIG. 5 that lead dioxide
(PbO.sub.2) used as an electrocatalyst in any acidic aqueous
environment having a pH of about 1 is only stable under anodic
electrode potentials corresponding to ozone evolution. At
potentials more negative than approximately 1.500 V (SHE), lead
dioxide (PbO.sub.2) dissolves to give Pb.sup.2+ cations. However,
at potentials more negative than approximately -0.350 V (SHE), lead
dioxide (PbO.sub.2) is electrochemically reduced directly to lead
(Pb) metal, which is then stable under these negative electrode
potentials.
[0017] It is clear from these illustrations that for an
electrochemical cell comprising copper cathodic
electrocatalyst/hydrated proton exchange membrane in the proton
form/lead dioxide anodic electrocatalyst, both electrocatalysts
will only be stable and useful when a sufficient cell voltage is
applied across the cell to give an anodic electrode potential more
positive than about 1.500 V (SHE) at the lead dioxide/hydrated
proton exchange membrane interface and a cathodic electrode
potential more negative than about 0.000 V (SHE) at the
copper/hydrated proton exchange membrane interface. However, for
this electrochemical cell, and some other related electrochemical
cells having a hydrated proton exchange membrane in the acidic
proton form, problems arise when, for example, a DC voltage of
sufficient magnitude from a DC power supply is not being applied to
the cell. This situation arises, for instance, when the cell is
being shipped or stored while not in use. While this may not be a
problem for many electrocatalysts, other electrocatalysts degrade,
dissolve, corrode, or otherwise become inactive when exposed to the
acidic surface of the membrane in the absence of a suitably applied
electrode potential, such as during shipping or storage of the
electrochemical cell. For example, in an electrochemical cell that
produces ozone, lead dioxide (PbO.sub.2) is a preferred
electrocatalyst for the anode electrode and platinum either in the
form of unsupported platinum black or as finely divided platinum
supported on any electronically conducting, high surface area
support, e.g., carbon or graphite powder, is a preferred
electrocatalyst for the cathode electrode. When the lead dioxide
(PbO.sub.2) is left exposed to the acidic surface of the hydrated
proton exchange membrane in the acidic proton form in the absence
of a suitably applied electrode potential during shipping and
storage, the lead dioxide electrocatalyst degrades, dissolves, or
is otherwise deactivated as described above, thereby losing some,
or all, of its ability to produce ozone from water.
[0018] Therefore, a method for storing membrane and electrode
assemblies or fully assembled electrochemical cells, or cell
stacks, that protect the electrocatalysts from the acidic surfaces
of PEMs during shipping and storage would be desirable. It would
also be desirable if the method could be applied to electrochemical
cells, or electrochemical cell stacks, that are being taken out of
service and stored for a period. Furthermore, it would be desirable
if the method protected the PEM itself from damage, which may occur
if the PEM is allowed to dry out after assembly.
SUMMARY OF THE INVENTION
[0019] The present invention provides a method for preparing or
storing membrane and electrode assemblies used in electrochemical
cells, or electrochemical cell stacks, where the membrane is
capable of conducting protons or other cations. The method
comprises providing an electrochemical cell having one or more
electrocatalysts in intimate contact with a cation exchange
membrane in a non-proton form. The non-proton form of the cation
exchange membrane may be selected from, for example, an alkali
metal cation or an ammonium cation, including tetraalkylammonium
cations. Also, the membrane may be in the form corresponding to the
precursor of a cation exchange membrane, such as the sulfonyl
fluoride form. Alternatively, the membrane may be in the acidic
proton form but in this case must be maintained in the dry proton
form, preferably by sealing the membrane from the surrounding
environment because the membrane is sufficiently hygroscopic to
take up moisture from the air. The method of the present invention
helps protect those electrocatalysts that are susceptible to being
damaged, dissolved, corroded, or otherwise deactivated when in
contact with the acid membrane surface of a hydrated cation
exchange membrane in the proton form, such as when no suitably
applied DC voltage or no DC electrical current of sufficient
magnitude is maintained through the electrochemical cell. An
example of such an electrocatalyst is lead dioxide (PbO.sub.2),
which can exist in two crystallographic forms, alpha lead dioxide
and beta lead dioxide.
[0020] The method further comprises supplying the electrochemical
cell with reactants and providing an electrical current through the
electrochemical cell, to liberate protons from the reactants, and
convert the cation exchange membrane from, for example, the alkali
metal cation form to a wet acidic proton form.
[0021] Furthermore, when the membrane is in the sulfonylfluoride
precursor form to a cation exchange membrane, the method further
comprises contacting the precursor sulfonyl fluoride form of the
membrane with an alkali metal hydroxide solution, wherein the
sulfonyl fluoride membrane is converted into the alkali metal
cation conducting form. The alkali metal hydroxide may be selected
from NaOH, KOH, LiOH, RbOH, CsOH, FrOH, and combinations thereof,
having a concentration of between about 0.1 M and about 10 M,
preferably between about 0.5 M and about 5 M, and most preferably
between about 0.75 M and about 3 M. The alkali metal hydroxide
solution may contact the sulfonyl fluoride form of the membrane for
a set period, such as between about 0.25 hours and about 24 hours,
preferably between about 0.5 hours and about 12 hours or most
preferably between about 1 hour and about 6 hours.
[0022] In another embodiment of the present invention, the method
comprises, prior to removing a DC electrical potential or voltage
from across an electrochemical cell, or cell stack, having one or
more electrocatalysts in contact with a cation exchange membrane in
a wet acidic proton form, converting the cation exchange membrane
from the acidic proton form to the alkali metal cation form with an
alkali metal hydroxide solution. The alkali metal hydroxide may be
selected from NaOH, KOH, LiOH, RbOH, CsOH, FrOH and combinations
thereof, having a concentration of between about 0.1 M and about 10
M, preferably between about 0.5 M and about 5 M, and most
preferably between about 0.75 M and about 3 M. The alkali metal
hydroxide solution may contact the proton exchange membrane for a
set period, such as between about 0.25 hours and about 24 hours,
preferably between about 0.5 hours and about 12 hours or most
preferably between about 1 hour and about 6 hours. The
electrocatalyst may be an anodic electrocatalyst that could be
damaged, dissolved, corroded, or otherwise deactivated when in
contact with an acid, such as, for example, lead dioxide.
[0023] The method further comprises draining the alkali metal
hydroxide solution from the electrochemical cell, or cell stack,
and further supplying the electrochemical cell with reactants and
restoring the DC electrical potential or voltage across the
electrochemical cell, to liberate protons from the reactants and
convert the alkali metal cation form of the cation exchange
membrane back to the acidic proton form.
[0024] The present invention also provides a method that may be
used with membrane and electrode assemblies comprising providing a
membrane and electrode assembly having one or more electrocatalysts
in intimate contact with a cation exchange membrane in a non-proton
form. The form of the cation exchange membrane may be selected
from, for example, an alkali metal cation form or an ammonium
cation form, including tetraalkylammonium cations. Also, the
membrane may be in the form corresponding to the precursor of a
cation exchange membrane, such as the sulfonyl fluoride form. The
membrane and electrode assembly may be assembled into an
electrochemical cell, such as an electrolytic cell, or a stack of
electrolytic cells. The electrocatalyst used in the method of the
present invention may be an anodic electrocatalyst, such as lead
dioxide, that could be damaged, dissolved, corroded, or otherwise
deactivated when in contact with an aqueous acid environment in the
absence of a suitably applied cell voltage, or cell stack
voltage.
[0025] The present invention also provides an electrochemical cell
having one or more electrocatalysts in intimate contact with a
cation exchange membrane in a non-proton form. The non-proton form
may be selected from, for example, an alkali metal cation and an
ammonium cation, including tetraalkylammonium cations. Also, the
membrane may be in a form corresponding to the precursor of a
cation exchange membrane, such as the sulfonyl fluoride form.
Alternatively, the cation exchange membrane may be in the proton
form but in this case must be maintained in the dry proton form
when there is no electrical current passing through the
electrochemical cell. The electrochemical cell may be one of a
plurality of electrochemical cells in an electrochemical cell
stack. At least one of the electrocatalysts degrades, dissolves,
corrodes, or is otherwise inactivated when placed in contact with a
wet acid proton form of the cation exchange membrane in the absence
of a suitable electrical current passing through the cell. An
example of such an electrocatalyst is an electrocatalyst that
includes lead dioxide, either alpha lead dioxide, beta lead
dioxide, or a combination of both alpha lead dioxide and beta lead
dioxide.
[0026] Additionally, the present invention provides a membrane and
electrode assembly comprising one or more electrocatalysts in
intimate contact with a cation exchange membrane in the non-proton
form. The non-proton form may be selected from, for example, an
alkali metal cation, or an ammonium cation, including
tetraalkylammonium cations. Also, the membrane may be in a form
corresponding to the precursor of a cation exchange membrane, such
as the sulfonyl fluoride form. The membrane and electrode assembly
may be assembled into an electrochemical cell, such as an
electrolytic cell, or a stack of electrolytic cells. At least one
of the electrocatalysts degrades, dissolves, corrodes, or is
otherwise inactivated when placed in contact with a wet acid proton
form of the cation exchange membrane in the absence of an
electrical current passing through the cell, or stack of cells. An
example of such an electrocatalyst is an electrocatalyst that
includes lead dioxide as a component.
[0027] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of a preferred embodiment of the invention, as
illustrated in the accompanying drawings, figures, and schematics
wherein like reference numbers represent like parts of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 shows the chemical formula of exemplary polymers used
in cation exchange polymer electrolyte membranes.
[0029] FIG. 2 is a schematic diagram showing the synthesis of a
sulfonyl fluoride polymer and the subsequent conversion of the
sulfonyl fluoride polymer into an active ionomer or cation exchange
polymer by base hydrolysis.
[0030] FIG. 3 is a schematic diagram of the commonly proposed
structure for perfluorosulfonic acid (PFSA) polymers.
[0031] FIG. 4 is an electrode potential--pH equilibrium diagram for
the system copper-water at 25.degree. C.
[0032] FIG. 5 is an electrode potential--pH equilibrium diagram for
the system lead-water at 25.degree. C.
[0033] FIG. 6 is a cross sectional view of a partially assembled
electrochemical cell stack.
[0034] FIG. 7 is a flowchart of a method for storing an
electrochemical cell stack having cation exchange membranes in an
alkali metal cation form.
[0035] FIG. 8 is a flowchart of a method for storing an
electrochemical cell stack having dry proton exchange membranes,
that is cation exchange membranes in the dry proton form.
[0036] FIG. 9 is a flowchart of a method for storing an
electrochemical cell stack by converting the cation exchange
membranes from the acidic proton form to the neutral alkali metal
cation form, or the salt form.
[0037] FIG. 10 is a comparative plot of cell voltage versus time,
and for Nafion.RTM. 105 in the hydrated proton form and in the
hydrated sodium cation form.
[0038] FIG. 11 is a comparative plot of weight percent ozone versus
time for Nafion.RTM. 105 in the hydrated proton form and in the
hydrated sodium cation form.
[0039] FIG. 12 is a comparative plot of cell voltage versus time
for Nafion.RTM. 1035 in the hydrated proton form, hydrated lithium
cation form, hydrated sodium cation form, and hydrated potassium
cation form.
[0040] FIG. 13 is a comparative plot of weight percent ozone versus
time for Nafion.RTM. 1035 in the hydrated proton form, hydrated
lithium cation form, hydrated sodium cation form, and hydrated
potassium cation form.
[0041] FIG. 14 is a comparative plot of cell voltage versus time
for Nafion.RTM. 117 in the proton form and in the sodium cation
form for a ten cell stack.
[0042] FIG. 15 is a comparative plot of weight percent ozone versus
time for Nafion.RTM. 117 in the proton form and in the sodium
cation form for a ten cell stack.
[0043] FIG. 16 is a comparative plot of cell voltage versus time
for Nafion.RTM. 117 in the proton form and in the sodium cation
form for a five cell stack.
[0044] FIG. 17 is a comparative plot of weight percent ozone versus
time for Nafion.RTM. 117 in the proton form and in the sodium
cation form for the five cell stack.
DETAILED DESCRIPTION
[0045] The present invention provides a method for preparing or
storing a membrane and electrode assembly having a solvated solid
polymer electrolyte membrane in contact with an electrocatalyst
without causing dissolution, corrosion, or inactivation damage to
the electrocatalyst. The method comprises providing one or more
electrocatalysts in intimate contact with a hydrated cation
exchange membrane in a form selected from an alkali metal cation
form, and an ammonium cation form, without continuously providing
an electrode potential, or voltage, across the
membrane/electrocatalyst interface. In another embodiment, the
method comprises providing one or more electrocatalysts in contact
with a dry cation exchange membrane in a form selected from an
alkali metal cation form, an ammonium cation form, and a proton
form. In a further embodiment, the method comprises providing one
or more electrocatalysts in contact with a dry or wet sulfonyl
fluoride polymer membrane precursor to a cation exchange polymer
electrolyte membrane. It should also be recognized that a
combination of embodiments may be used throughout the life of an
MEA, electrochemical cell or cell stack. For example, an MEA may be
shipped with a cation exchange membrane in a dry proton form, but
following a first use the MEA may be suitably stored in an alkali
metal cation form to avoid having to dry the membrane.
[0046] When the cation exchange membrane is in the proton form and
becomes hydrated, the resulting acidity of the cation exchange
membrane surface can degrade, dissolve, corrode, or otherwise
inactivate some electrocatalysts in contact with the cation
exchange membrane in the absence of a DC current flowing through
the cation exchange membrane/electrocatalyst interface. For
example, the lead dioxide electrocatalyst that is often used in the
electrochemical production of ozone from water degrades, dissolves,
corrodes, or is otherwise inactivated if the electrocatalyst
remains in contact with the hydrated cation exchange membrane in
the proton form without maintaining at least a trickle current
across the cation exchange membrane/lead dioxide interface. The
lead dioxide electrocatalyst material is not degraded in this
manner, however, if the cation exchange membrane is not acidic. The
cation exchange membrane is not acidic when the membrane is in the
alkali metal cation form or in the ammonium cation form. Also, a
cation exchange membrane, even when in the proton form, is not
effectively acidic if the membrane is dry, that is neither solvated
or hydrated. Further, a dry or wet sulfonyl fluoride polymer
membrane precursor to a cation exchange polymer electrolyte
membrane is not acidic.
[0047] FIG. 6 is a cross sectional view of a partially assembled
electrochemical cell. The electrochemical cell may be, for example,
an electrolytic cell (e.g., an electrolyzer) or a fuel cell. This
exemplary electrochemical cell comprises a cation exchange membrane
101 having a cathode electrocatalyst 104 and an anode
electrocatalyst 103 on either side of the cation exchange membrane
101. The membrane 101 is disposed within two cell frames 105, 106
that hold the membrane 101 in place. Alternate means may be used to
secure the membrane 101 such as using an adhesive. An anode flow
field 108 and a cathode flow field 107 deliver reactants to the
electrocatalysts. Reactant fluids are supplied to the flow fields
through manifolds, such as the cathode manifold 109. Separator
plates 110, 111 may be used to separate a plurality of
electrochemical cells from one another to form a stack.
[0048] As may be seen from FIG. 6, the electrocatalysts 103, 104
are in intimate contact with the cation exchange membrane 101,
collectively forming a membrane and electrode assembly (MEA). For
some electrochemical cells, in particular fuel cells, the MEA is
typically an integral member, surrounded by other components that
make it impossible to separate the electrocatalysts from the acidic
proton exchange membrane by, for example, inserting a temporary
mechanical barrier between the catalysts and the proton exchange
membrane. Therefore, to prevent damage, dissolution, corrosion, or
other means of inactivation of the electrocatalyst, the present
invention provides for assembling an MEA or an electrochemical cell
using a hydrated cation exchange membrane in the alkali metal
cation form or in the ammonium cation form. Alternatively, the MEA
or the electrochemical cell may be assembled with a dry cation
exchange membrane in the proton form, in the alkali metal cation
form, or in the ammonium cation form. Another alternative is to
assemble the MEA or the electrochemical cell using a dry or wet
sulfonyl fluoride polymer membrane, which is ionically
non-conducting, but which is a precursor to a cation exchange
polymer electrolyte membrane. Any of these alternatives will
protect the electrocatalyst in intimate contact with the cation
exchange membrane from degrading, dissolving, corroding, or
otherwise being inactivated while there is no current flowing
through the MEA or the electrochemical cell.
[0049] Because the cation exchange membrane swells as it takes up
water (or on being solvated) and shrinks as it dries out, it is
generally preferred to assemble an electrochemical cell while the
cation exchange membrane is wet and to maintain that wetness while
storing the electrochemical cell. By maintaining the cation
exchange membrane in a wet condition, that is hydrated (or
solvated), stresses generated by the drying out and shrinking of
the cation exchange membrane may be avoided. These stresses may
cause the cation exchange membrane to weaken and possibly even tear
or crack, especially where the membrane contacts frames or other
features that hold portions of the membrane in place. In addition,
if the membrane is held in place using an adhesive, the swelling
and shrinking of the membrane, caused by drying out the membrane
and then rehydrating the membrane, will place additional strains on
the adhesive bonds.
[0050] To convert the polymer membrane from its ionically
non-conducting sulfonyl-fluoride form to the alkali metal cation
form, the sulfonyl fluoride polymer membrane is contacted with an
alkali metal hydroxide solution. The preferred solvent for such
solutions is water. However, mixed aqueous-nonaqueous solvents,
e.g., water-methanol, or nonaqueous solvents, e.g., methanol, are
also applicable. The alkali metal hydroxide solutions have alkali
metal cations selected from Li, K, Na, Rb, Cs, Fr and combinations
thereof. The concentration of the alkali metal hydroxide may be
between about 0.1 M and about 10 M, preferably between about 0.5 M
and about 5 M, and most preferably between about 0.75 M and about 3
M. The alkali metal hydroxide solution may contact the sulfonyl
fluoride polymer membrane for a set period, such as between about
0.25 hours and about 24 hours, preferably between about 0.5 hours
and about 12 hours or most preferably between about 1 hour and
about 6 hours. After being contacted with the alkali metal
hydroxide solution, the newly formed cation exchange polymer
membrane should then be washed with water to remove any excess
alkali metal hydroxide. The cation exchange polymer membrane may
then be used to assemble an MEA or an electrochemical cell while
the cation exchange membrane is in the alkali metal cation
form.
[0051] To convert a cation exchange polymer membrane chemically
from the alkali metal cation form to the acidic proton form, the
cation exchange membrane may be contacted with a strong acid
solution. The preferred solvent for such solutions is water.
However, mixed aqueous-nonaqueous solvents, e.g., water-methanol,
or nonaqueous solvents, e.g., methanol, are also applicable. The
strong acid may be selected, for example, from HNO.sub.3, HCl,
H.sub.2SO.sub.4, and combinations thereof. The concentration of the
strong acid may be between about 0.1 M and about 10 M, preferably
between about 0.5 M and about 5 M, and most preferably between
about 1 M and about 3 M. The strong acid solution may contact the
cation exchange membrane for a set period, such as between about
0.25 hours and about 24 hours, preferably between about 0.5 hours
and about 12 hours or most preferably between about 1 hour and
about 6 hours.
[0052] Alternatively, if the cation exchange membrane is contained
within an electrochemical cell, the cation exchange membrane may be
converted from the alkali metal cation form to the acidic proton
form simply by placing the electrochemical cell or cell stack in
operation. By supplying reactants to the electrochemical cell and
applying a DC voltage across the cell, the cation exchange membrane
may be converted from the alkali metal cation form to the acidic
proton form electrochemically as the alkali metal cations on the
surface and within the bulk of the membrane are driven out and
replaced with protons generated by electrochemical reactions at the
anode involving the reactants.
[0053] The present invention is suitable for assembling and storing
any type of MEA, electrochemical cell, or electrochemical cell
stack having an electrocatalyst that may be degraded or damaged
arising from dissolution, corrosion, or other inactivation
processes when in contact with the surface of a hydrated cation
exchange membrane having an acidic surface containing protons.
While most of the present discussion is directed to ozone
generators using PFSA polymers as solid polymer electrolytes, the
present invention is not meant to be so limited. Other types of
electrocatalysts and solid polymer electrolyte membranes would
equally benefit from the application of the methods disclosed
herein.
[0054] FIG. 7 is a flow chart of a method for assembling an
electrochemical cell having a cation exchange membrane in an alkali
metal cation form, or ammonium cation form, in accordance with the
present invention. In state 150, a non-ionically conducting
sulfonyl fluoride polymer membrane is treated with a strong base
solution, such as 2M NaOH (or NH.sub.4OH) for about 3 hours, to
convert the membrane from the sulfonyl fluoride form to the alkali
metal cation form. Optionally, the membrane may then be rinsed with
distilled, or deionized, water to remove any excess NaOH (or
NH.sub.4OH). In state 152, the electrocatalysts are applied to the
surface of the cation exchange membrane, either as ink electrodes,
or by mechanically pressing an electrocatalyst-coated substrate.
One side of the membrane has the cathode electrocatalyst material
and the other side of the membrane has the anode electrocatalyst
material. Optionally, the electrocatalyst may be placed in intimate
contact with the cation exchange membrane without actually being
applied to the surface as an ink electrode. If an MEA is all that
is being prepared, then the process is finished, until the MEA is
to be placed into an electrochemical cell. In state 154, the
electrochemical cell, in this case an electrolytic cell for the
production of ozone, is assembled with all the components required
for operation of the cell or a cell stack. These components may
include, for example, the cation exchange membrane, the
electrocatalysts (for example, lead dioxide (PbO.sub.2) as the
anode electrocatalyst and unsupported platinum black, or finely
divided supported platinum, as the cathode electrocatalyst for
ozone generation service), the flow fields, separator plates,
frames and other required components. In state 156, the
electrochemical cell, or cell stack, is stored for a period until
the cell, or stack, is needed for service. Since the surface of the
cation exchange membrane was not acidic during the storage period,
the lead dioxide (PbO.sub.2) electrocatalyst that was in intimate
contact with the membrane will not be harmed.
[0055] In state 158, when the storage period has ended, the anode
of the electrochemical cell, or cell stack, is provided with water
and a potential or voltage of sufficient magnitude is applied
across the cell or cell stack to start the electrochemical reaction
that produces ozone. In state 160, the cation exchange membrane
that was stored in the alkali metal cation (sodium cations) form or
state is electrochemically converted to the acidic proton form
while producing ozone from water. It should be noted that there may
be a slight increase in cell voltage while the alkali metal cations
(sodium cations) are being driven out of the membrane and replaced
with protons. However, most, if not all, of the alkali metal
cations (sodium cations) will be replaced with protons over time.
Alternatively, the cation exchange membrane in the alkali metal
cation (sodium cations) form could have been converted to the
acidic proton form by contacting the membrane with a strong acid
solution for a period as described above, preferably in the
presence of an electrical current flowing through the cell.
[0056] FIG. 8 is a flowchart of a method for storing an
electrochemical cell having a dry cation exchange membrane in the
proton form in accordance with the present invention. In state 180,
a sulfonyl fluoride polymer membrane is treated with a strong
alkali metal hydroxide solution to convert the sulfonyl fluoride
polymer membrane to a cation exchange membrane in the alkali metal
cation form and subsequently with a strong acid solution to convert
the cation exchange membrane in the alkali metal cation form to the
acidic proton form of the membrane. Optionally, the cation exchange
membrane may be washed in water after the base and acid treatments
to remove excess hydroxide and acid from the cation exchange
membrane. In state 182, the cation exchange membrane in the acidic
proton form is allowed to dry out. By drying out the cation
exchange membrane in the proton form, the surfaces are no longer
acidic since water molecules for hydration of the protons are no
longer available and therefore, the electrocatalysts will not be
damaged or deactivated when placed in contact with the surfaces. In
state 184, an electrocatalyst is applied to at least one side of
the dry cation exchange membrane in the proton form. Preferably,
one side of the membrane has the cathode electrocatalyst and the
other side of the membrane has the anode electrocatalyst.
Optionally, one or more of the electrocatalysts may be placed in
intimate contact with the cation exchange membrane in the proton
form without actually being applied to the surface as ink
electrodes. In state 186, the electrochemical cell, in this case an
electrochemical cell stack to produce ozone, is assembled with all
the components of the stack, including the dry cation exchange
membrane in the proton form, the electrocatalysts (for example,
lead dioxide (PbO.sub.2) as the anode electrocatalyst and platinum
black as the cathode electrocatalyst for ozone generation service),
the flow fields, separator plates, frames and other required
components. In state 188, the electrochemical cell is stored for a
period until the cell is required for service. During this storage
period, the dry surface of the cation exchange membrane in the
proton form is not acidic, and therefore, the lead dioxide
(PbO.sub.2) electrocatalyst will not be harmed since the lead
dioxide (PbO.sub.2) is not in contact with a hydrated acidic
surface. In state 190, the electrochemical cell, or cell stack, is
provided with water to allow the cation exchange membranes in the
proton form to be become wet, as required to operate the
electrochemical cell, or cell stack. In state 192, the
electrochemical cell, or cell stack, is started up by applying a DC
voltage of sufficient magnitude to induce a current to flow through
the cell, or cell stack, giving rise to ozone evolution. It is
preferable that steps 190 and 192 occur almost simultaneously. It
should be recognized that states 180 and 182 may be eliminated if
the cation exchange membrane is already provided in the dry proton
form.
[0057] FIG. 9 is a flowchart of a method for storing an
electrochemical cell comprising converting the cation exchange
membrane from the acidic proton form to the alkali metal cation
form. This method may be used when, for example, an electrochemical
cell, or cell stack, is removed from service for storage. In state
210, the operating electrochemical cell, or cell stack, for
example, an ozone generator, is filled with an alkali metal
hydroxide solution so that the cation exchange membranes in the
proton form are contacted with the hydroxide solution. In state
212, the alkali metal hydroxide solution is allowed to remain in
contact with the cation exchange membrane in the proton form within
the electrochemical cell, or cell stack, for a set or variable
period. This period is preferably between about 1 and 6 hours.
Current flowing through the cell, or cell stack, associated with
the DC power applied to the cell helps to replace the protons
within the cation exchange membrane with the alkali metal cations.
In state 214, the alkali metal hydroxide solution is drained from
the electrochemical cell, or cell stack, and the cell, or cell
stack, is stored with the cation exchange membranes in the alkali
metal cation form remaining wet, although alternatively, the
membranes may be allowed to dry out. The advantage of keeping the
membranes wet is that the membranes are not placed under the stress
that occurs due to the shrinkage and swelling during the dry-out
and rehydration process. During this storage period, the surface of
the cation exchange membrane is not acidic since the membrane is in
the alkali metal cation form. In state 216, when the
electrochemical cell, or cell stack, is to be placed back into
service, the anode of the electrochemical cell, or cell stack, is
filled with water. In state 218, DC electrical power is applied to
the electrochemical cell, or cell stack, and, in state 220, the
cation exchange membrane in the alkali metal cation form converts
back to the acidic proton form as ozone is produced.
EXAMPLES
[0058] In the first set of experiments, a single electrochemical
cell having an active area of 25 cm.sup.2 was used. The cell was
placed electrically in series with a DC power supply, and an
ammeter. Also, a DC voltmeter was connected to terminals which were
attached to the positive and negative endplate of the single cell.
The input to the anode compartment of the electrochemical cell was
connected fluidically in series with a heat exchanger, a pump, and
a water reservoir, where the latter also functioned as a gas/liquid
phase separator. The output from the anode compartment of the
single cell was connected to the input of the water reservoir/phase
separator. The phase separated anode gas stream was connected to an
ozone monitor (Ozone Research and Equipment Corporation, model
03M-110) which was used for measuring gas-phase ozone
concentrations. The ozone-containing gas stream was separated from
the fluid flow exiting from the anodic outlet port in the water/gas
separator at atmospheric pressure. The gas stream was then fed into
the inlet of the ozone monitor under atmospheric pressure. This
instrument measures the absorption of UV light by ozone at 252 nm
and provides a direct determination of ozone concentrations in
terms of mg/standard liter or mg min.sup.-1. The output from the
cathode compartment of the electrochemical cell was connected to a
vessel where the evolved hydrogen gas was phase separated from the
electroosmotically transported water and vented. In one experiment
where the cation exchange membrane was initially in the sodium
cation form, the first 25 milliliters of electroosmotically
transported water collected from the cathode compartment had a pH
of 12.2.
[0059] The anode electrodes comprised a porous titanium substrate
which was first electroplated with a very thin layer of platinum
and subsequently electroplated with lead dioxide (PbO.sub.2). The
cathode electrodes comprised porous SS316 felt which also was
electroplated with a thin layer of platinum. Membrane and electrode
assemblies (MEAs) were assembled on mechanically pressing the lead
dioxide-coated anode electrode in intimate contact with one side of
a cation exchange membrane and intimately contacting the other side
of the membrane with the platinum-plated cathode electrode. An
endplate was placed behind each electrode substrate which were
subsequently bolted together so as to keep the single cell under
compression. The inner face of each endplate had machined into it
serpentine flow fields to provide a means of supplying reactants
and removing products from both the anode compartment and the
cathode compartment of the cell. Gaskets of appropriate size and
thickness encompassed the perimeters of the anode electrode and the
cathode electrode so as to provide sealing for the single cell.
Cation exchange membranes used consisted of Nafion.RTM. 105 (5 mils
thick) and Nafion.RTM. 1035 (3.5 mils thick), both in the hydrated
proton form and in the hydrated alkali metal cation form.
[0060] To place a cell in operating condition, deionized water from
the water reservoir was pumped through the heat exchanger followed
by the anode compartment of the cell and returned to the water
reservoir/phase separator. The temperature of the output water from
the anode compartment was maintained at 25.+-.1.degree. C. After
one-to-two minutes of circulating water, a constant current density
of 1.6 A cm.sup.2 of active electrode area was impressed and
maintained on the electrochemical cell. At various times both the
cell voltage and the weight percent ozone in the phase separated
gas stream from the anode compartment were measured. Comparative
plots of cell voltage versus time, and weight percent ozone versus
time, for Nafion.RTM. 105 in the hydrated proton form and in the
hydrated sodium cation form are presented in FIGS. 10 and 11,
respectively. Similarly, comparative plots of cell voltage versus
time, and weight percent ozone versus time, for Nafion.RTM. 1035 in
the hydrated proton form, hydrated lithium cation form, hydrated
sodium cation form, and hydrated potassium cation form are
presented in FIGS. 12 and 13, respectively. Clearly, the ability to
start-up a cell is not affected by the nature of the mobile cation
initially present in the cation exchange membrane solid polymer
electrolyte. Maintaining electrochemical performance of a cell
(cell voltage and weight percent ozone) is also not affected by the
nature of the cation exchange membrane used to fabricate the
membrane and electrode assembly.
[0061] In the second set of experiments, an electrochemical cell
stack, comprising either five cells or ten cells electrically in
series and having an active area of 6 cm.sup.2 per cell, was placed
in series with a DC power supply, and an ammeter. Also, a DC
voltmeter was placed across each of the cells. The anode
compartments of the electrochemical cell stack were connected
fluidically to a solenoid valve to allow deionized make-up water to
enter the stack. The solenoid valve was controlled by a level
sensor that monitored the water level in the anode compartments.
The anode compartments in the stack also functioned as a gas/liquid
phase separator for the ozone/oxygen output mixture. The cathode
compartments of the cell stack were connected in series with a heat
exchanger, a pump, and a water reservoir, where the latter also
functioned as a gas/liquid phase separator of evolved hydrogen from
the circulating cathode water stream. The individual cells in the
cell stack were identical and assembled electrically in series and
fluidically in parallel.
[0062] The anode electrodes comprised porous titanium substrates,
which were first plated with a very thin layer of platinum and
subsequently electroplated with lead dioxide (PbO.sub.2). The
cathode electrodes comprised porous 316SS felt. A cell was
assembled with an anode cell assembly, cathode cell assembly,
cation exchange membrane, and bipolar plate. The anode cell
assembly comprised an anode electrode placed on top of the anode
flow field. The anode flow field was a built-up assembly of
expanded titanium screens with perpendicular expansions and was
coated with a proprietary conversion coating to maintain low
electrical resistance. The anode electrode and anode flow field
were placed inside the anode cell frame. The cathode cell assembly
was comprised of a cathode electrode placed on top of the cathode
flow field. The cathode flow field was a built-up assembly of
expanded 316SS screens with perpendicular expansions. The cathode
electrode and cathode flow field were placed inside the cathode
cell frame. The cation exchange membrane was sandwiched between the
anode cell assembly and the cathode cell assembly such that the
electrodes were in intimate contact with opposing sides of the
cation exchange membrane. These cell assemblies were then "stacked"
on top of an endplate, separated with bipolar plates, and topped
with another endplate. Gaskets were placed between all parts to
provide sealing. The assembled stack was compressed with tie rods
spanning endplate to endplate to provide the compression needed for
electrical contact and sealing. The bipolar plates were titanium
and were coated with a proprietary conversion coating to maintain
low electrical resistance. The cation exchange membrane used was
Nafion.RTM. 117 (7 mils thick), both in the proton form and in the
sodium cation form.
[0063] To place a stack in operating condition, the cathode water
reservoir was filled with deionized water and pumped through the
cathode compartment of each cell in the stack to maintain the stack
at any desired temperature, for example, 25+/-2 degrees C. The
anode compartment of each cell in the stack was also filled with
deionized water and monitored using the float switch. After a few
minutes of anode water circulation, a constant current density of
3.0 A cm.sup.2 active electrode area was impressed and maintained
on the cell stack. At various times both the individual cell
voltages and the weight percent ozone produced on the anode side of
the cells were measured. Comparative plots of cell voltage versus
time and weight percent ozone versus time for Nafion.RTM.117 in the
proton form and in the sodium cation form are presented in FIGS. 14
and 15, respectively, for the ten cell stack and in FIGS. 16 and
17, respectively, for the five cell stack. Again, the start-up of
the electrochemical cell stack and the electrochemical performance
(cell voltage and weight percent ozone) of the stack was not
affected by the nature of the cation exchange membrane used to
fabricate the membrane and electrode assemblies.
[0064] It will be understood from the foregoing description that
various modifications and changes may be made in the preferred
embodiment of the present invention without departing from its true
spirit. It is intended that this description is for purposes of
illustration only and should not be construed in a limiting sense.
The scope of this invention should be limited only by the language
of the following claims.
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